Question: Which of these numbers is prime? ${25,\ 31,\ 35,\ 69,\ 84}$
A prime number has exactly two factors: 1 and itself. The factors of 25 are 1, 5, and 25. The factors of 31 are 1 and 31. The factors of 35 are 1, 5, 7, and 35. The factors of 69 are 1, 3, 23, and 69. The factors of 84 are 1, 2, 3, 4, 6, 7, 12, 14, 21, 28, 42, and 84. Thus, 31 is a prime number.